Land mobile radios providing two-way radio communication are utilized in many fields, such as law enforcement, public safety, rescue, security, trucking fleets, and taxi cab fleets to name a few. Land mobile radios include both vehicle-based and hand-held based units. Digital land mobile radios have additional processing inside the radio to convert the original analog voice into digital format before transmitting the signal in digital form over-the-air. The receiving radio receives the digital signal and converts it back into an analog signal so the user can hear the voice. Examples of digital radio are radios that comply with the APCO-25 standard or TETRA standard. However, digital radios have sometimes been perceived to distort certain speech sounds. In particular, speech sounds having alveolar trills, such as the rolled ‘r’ used in Spanish and Italian languages, can be perceived as sounding distorted, flat or slurred.
In radio operation, incoming audio speech into a microphone is converted by an analog-to digital (A/D) converter) resulting in digitized speech signal which is input to a vocoder. Narrowband vocoders are used in digital radio products. FIG. 1 is a graphical example 100 comparing pre-vocoder trill sounds to post-vocoder trill sounds in accordance with the prior art. Graphs 102 and 104 show time versus amplitude for two speech samples. Uncoded alveolar trills 106 and 110 (pre-vocoder) are shown in graph 102. Corresponding post-vocoder coded/decoded alveolar trills 108 and 112 are shown in graph 104. As shown in graph 104, the alveolar trills 108 and 112 are smeared and are thus not encoded correctly by the narrowband vocoder causing intelligibility problems, especially in Italian and Spanish. Because vocoders are typically regulated by the standard within which they operate, they cannot be easily modified.
Accordingly, a means to improve the fidelity of vocoded higher modulation rate speech sounds without modifying the vocoder is needed.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.